1. Field of the Invention
This application is related to the field of temperature sensing. In particular, this invention is related to a magnetic sensor in which the Curie magnetic transition of an amorphous ferromagnetic material is utilized, and a method of remote temperature sensing using the magnetic sensor.
2. Description of the Related Art
There are a number of techniques and tools available to measure temperature, including a well-known temperature indicator such as classical mercury thermometer, thermocouple, resistance thermometer, bi-metal, and the like. All of these utilize some basic physical phenomena which vary with temperature, thus each having a unique feature. For example, mercury thermometry is effective for visual sensing of temperature, but is not suited for direct conversion of temperature to an electrical signal. A thermocouple which utilizes thermoelectric effects of metals is more suited if an electronic reading of temperature is needed. However, a thermocouple has to be wired to a voltmeter which converts an electrical signal to a corresponding temperature. A resistance thermometer which utilizes the temperature dependence of resistivity of a metal also has to be wired to a voltmeter. These techniques require wire connections between a sensor and a temperature indictor, and therefore, are not suited for remote sensing of temperature. In such cases, as in sensing the temperature of a moving or a physically inaccessible object, remote sensing of temperature becomes necessary. This kind of a sensor must respond to temperature and send a temperature-dependent signal wirelessly to a detector for further signal processing. One such sensor may be realized by utilizing the Curie magnetic transition in a ferromagnetic material. A ferromagnetic material such as iron has a ferromagnetic Curie temperature, above which ferromagnetism disappears along with all related phenomena such as high magnetization and permeability. The change of the magnetization and the permeability of a ferromagnetic material at the Curie temperature may be readily detected remotely by a conventional magnetometry. Using this technique, a temperature-sensitive switch was disclosed in U.S. Pat. No. 4,434,411 (hereinafter, the '411 patent). The '411 patent utilizes the change at the Curie temperature of the magnetic force acting on the magnetic materials attached to a mechanical switch, providing a temperature-sensitive switch which replaces a conventional bimetal-based switch.
A remote temperature sensing capability is increasingly in demand in office, home and industry, in which more electronics are being utilized to control equipment and machinery. One such example is disclosed in U.S. Pat. No. 4,052,696 (hereinafter, the '696 patent), in which the Curie magnetic transition of a ferrite is utilized in a tire temperature monitor. This technique, however, requires a very small gap between the ferrite-based sensor and a stationary detector to maintain a reliable detecting signal. The width of this gap is too small because ferrites usually have relatively low magnetic permeabilities ranging from 80 to 2,000 [see, for example, Table 22-2 on page 498 of “Physics of Magnetism” by S. Chikazumi (John Wiley & Sons, NY, 1964)]. In addition, the availability of different Curie temperatures suited for this kind of temperature sensing from ferrite products is limited to several discrete temperatures ranging from 80° C. to 130° C. as given in Table 22-2 of Chikazumi's book. Furthermore, a selected ferrite with a predetermined magnetic Curie transition temperature is a part of an inductive circuit of the sensor, requiring several separate copper windings, as shown in FIGS. 1 and 2 of the '696 patent. Such electrical circuitry becomes cumbersome and needs to be avoided. A simpler, but more effective remote temperature sensing method is clearly needed.